1. Field of the Invention
This invention relates to an ink-jet printing method and apparatus, and in particular to an ink-jet-printing method and apparatus that prints an image using an ink droplet discharge head.
2. Description of the Background Art
As an ink-jet-printer, a serial type printer that forms and prints an image by reciprocating a carriage incorporating an ink droplet discharge head, and an ink droplet-on-demand type printer that executes printing with an ink droplet ejection head aligned thereon are known.
In such ink-jet-printers, when a half tone image is printed, a common driving waveform formed from multiple driving signals (i.e., multiple driving pulses) is generated per printing cycle (or driving cycle), and one or multiple drive signals are chosen from the common driving waveform and applied to a pressure generator (for example, an actuator) that discharges ink droplets from the ink droplet discharge head. Specifically, by discharging the same or different sized ink droplets from the ink droplet discharge head and either combining these ink droplets during their flights or shooting multiple ink droplets at the same ink droplet landing position, dots of a different size are formed.
However, a problem is that density of ink and consequently a printing image varies in these ink-jet printers as ambient temperature changes. Therefore, a constant density has been demanded in the past regardless of the change in the ambient temperature, and various technologies have been developed to resolve such a problem.
For example, as described in Japanese Patent No. 3674248 (JP-3674248-B), in an ink-jet printer, a temperature range is divided into several groups and different multiple driving waveforms are assigned to each groups. Then, by changing the voltage in accordance with the change in temperature in each of the prescribed temperature groups, an image is printed with almost constant density regardless of the change of temperature.
Further, as described in Japanese Patent Application Publication No. 2002-211011 (JP-2002-211011-A), a dot diameter changing device is provided in an ink-jet printer to change the number of discharge ink droplets and accordingly a pixel dot size. Specifically, the diameter changing device changes the number of discharged ink droplets depending on the environment and equalizes the dot size.
Yet there are problem with the above-described printers. For example, with the ink-jet printer of JP-3674248-B, as described above a different driving waveform is obtained by changing the voltage in each of the divided temperature ranges. However, fine adjustment is needed to obtain the waveform assigned to each of the divided temperature ranges and accordingly requires a considerable time period. Further, the ink-jet printer of JP-2002-211011-A demonstrates a system in which a discharging ink droplet is added to a rear side of a previously discharged ink droplet. Thus, as it is discharged later the ink droplet velocity needs to be accelerated to catch up with and combine multiple preceding ink droplets with each other as a single ink droplet. Accordingly, when the number of ink droplets increases, a waveform producing the last ink droplet becomes significantly large. As a result, an ink meniscus is likely disturbed, resulting in unstable discharging of ink droplets as a problem.